Umm, it’s easy and also the most sensible. Tell them to speak to DynoRod.
You can bet your arse is caused by roots.
Prior to taking the job on you should’ve checked the downpipes for blockages. It takes seconds, literally.
How do you check a cast iron downpipe for blockages when it’s 3+ floors up?
It'll be blocked from the ground up, tap the pipe, it'll not sound hollow.
Doesn’t work in my experience on the original cast iron pipes. They are usually so thick and well installed that they all sound the same, blocked or not.
I’ve walked away from a job exactly like this one today, large Georgianplace with 8 downpours most of them spilling over. Tap them with a spanner they all sound the same.
I’m glad this post was made anyway because without it I’d have priced and got this job and it probably would’ve ended up being a nightmare.